Heretofore, it has been known that a soft contact lens is softer than a hard contact lens and that when a soft contact lens is subjected to measurement while being held on a lens receiving plate in air, the shape thereof is apt to be deformed by its own weight.
Besides, a large amount of water is contained in the soft contact lens, so if the soft contact lens is allowed to stand in air for a long time, the water contained therein will evaporate. Consequently, if optical characteristics of the soft contact lens are measured in air by means of a lens meter, an error is apt to occur in measured values. To avoid this inconvenience, both skill and rapidness of measurement are required for the measurement of a soft contact lens.
To meet this requirement, a lens meter is being developed which measures optical characteristic values of a soft contact lens while dipping the contact lens in liquid to retain the shape thereof.
Since this lens meter can measure optical characteristic values while retaining the shape of a soft contact lens and without evaporation of water, skill is not so strictly required, nor is required so high rapidness, in the measurement.
However, optical characteristic values of a soft contact lens measured in liquid and those measured in air are different. In more particular terms, when measurement is made in liquid, the difference between the refractive index of a soft contact lens and that of the liquid is smaller than the difference between the refractive index of the soft contact lens and that of air. Consequently, optical characteristic values of a soft contact lens measured in liquid are smaller than those of the same contact lens measured in air.
Therefore, when optical characteristic values of a soft contact lens are measured in liquid, it is necessary to convert them into optical characteristic values in air, and the refractive index of the material of the soft contact lens is needed for the said conversion.
However, the refractive index of the material of a soft contact lens is unknown in many cases, so it is impossible to make conversion from optical characteristic values of a contact lens measured in liquid into optical characteristic values thereof in air which are optical characteristic values obtained when the contact lens is applied to an eye, and there exists a problem that converted optical characteristic values obtained by the conversion of optical characteristic values of a contact lens measured in liquid are poor in reliability.
The present invention has been accomplished in view of the above-mentioned circumstances and it is an object of the invention to provide a refractive power measuring method and apparatus able to measure optical characteristic values of a soft contact lens precisely in air.